The Peter McVerry Trust is calling for a transition plan to be put in place to prevent 100 people from finding themselves back on the streets.
The Bru Aimsir hostel which was set up last winter to provide emergency beds for homeless people is due to close in coming days.
The charity is calling for an postponement of the closure; CEO Pat Doyle said it was important to provide alternatives.
He said: "Even if it can’t stay open forever (there should be) an orderly and planned wind-down of the service. That would allow (us all) to look at other options for provision and look at other facilities.
"But to take out 100 beds overnight now - the impact of that will be fully borne by homeless people."